The present invention relates to a microscope objective and, more particularly, to an apochromatic microscope objective which has a high numerical aperture and yet which can be effectively corrected for various aberrations caused by a change in thickness of a transparent plane-parallel plate, e.g., a cover glass, which is disposed on the object side of the microscope objective.
In general, a microscope objective is designed on the assumption that the cover glass thickness is constant. Therefore, when the cover glass thickness changes, the imaging performance of the microscope objective deteriorates. This phenomenon becomes more remarkable as the numerical aperture (NA) increases. Particularly, when the NA is 0.8 or more, aberrations increase to a considerable extent with only a small change in the cover glass thickness, i.e., about 0.02 mm with respect to the design value. Since variation in thickness of commercially available cover glasses exceeds the above-described value, high-NA objectives other than oil immersion objectives necessitate devising appropriate measures to correct aberrations caused by a change in the cover glass thickness in order to ensure the required performance.
Incidentally, to correct variation of aberrations caused by a change in the cover glass thickness, a method has heretofore been adopted in which a part of a lens system constituting the objective is moved along the optical axis in accordance with a change in the cover glass thickness.
For example, Japanese Patent Application Laid-Open (KOKAI) Nos. 51-87056 (1976) and 61-275812 (1986) disclose an objective including a front lens unit for converting a divergent bundle of rays into a convergent bundle of rays, and a rear lens unit having a negative refractive power, wherein the rear lens unit is moved in accordance with a change in the cover glass thickness, thereby enabling aberration variation to be effectively corrected despite high NA.
Japanese Patent Application Post-Exam Publication Nos. 03-58492 (1991), 04-26445 (1992) and 04-26448 (1992) disclose a microscope objective which includes three lens units, that is, a 1-st lens unit having a positive refractive power, a 2-nd lens unit including a negative lens or having a small refractive power, and a 3-rd lens unit having a negative refractive power, wherein the 2-nd lens unit is moved along the optical axis, thereby correcting aberration variation caused by a change in the cover glass thickness. These conventional techniques correct aberration variation caused by a change in the cover glass thickness in the region of .+-.1 mm in examples having a NA not higher than 0.7.
In addition, Japanese Patent Application No. 03-281222 (1991) discloses an objective including four lens units, wherein the 2-nd lens unit, which has a small refractive power, is moved along the optical axis, thereby minimizing aberration variation caused by a change in the cover glass thickness, although the objective has a high NA.
As has been described above, Japanese Patent Application Laid-Open (KOKAI) Nos. 51-87056 (1976) and 61-275812 (1986) propose a technique of minimizing aberration variation caused by a change of the cover glass thickness in an objective having a high NA. However, the variation of chromatic aberration cannot satisfactorily be minimized simply by moving the rear lens unit in the lens system composed of two lens units.
Further, Japanese Patent Application No. 03-281222 (1991) proposes a technique of minimizing variation of chromatic aberration in an objective having a high NA. However, the proposed technique suffers from a complicated arrangement and a high cost.
Japanese Patent Application Post-Exam Publication Nos. 03-58492 (1991), 04-26445 (1992) and 04-26448 (1992) are disadvantageous in that the NA is small and chromatic aberration cannot satisfactorily be corrected.